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Bureau of Mines Information Circular/1985 



Metallurgical Effects of Impurities 
in Recycled Copper Alloys 




By Harry V. Makar and William D. Riley 




1 




UNITED STATES DEPARTMENT OF THE INTERIOR 



m 



T5I 

^^INES 75TH AV)''^ 



Information Circular 9033 



Metallurgical Effects of Impurities 
in Recycled Copper Alloys 

By Harry V. Makar and William D. Riley 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



4^ 






Library of Congress Cataloging in Publication Data; 



Makar, H. V. (Harry V.) 

Metallurgical effects of impurities in recycled copper alloys, 

(Information circular ; 9033) 

Bibliography: p. 18-19. 

Supt. of Docs, no.: I 28.27:9033. 

1. Copper alloy s— Metallurgy. 2. Copper alloy s— Inclusions. 3. 
Scrap metals. I. Riley, W. D. {William D.). II. Title. III. Series: 
Information circular (United States. Bureau of Mines) ; 9033. 



TN295.U4 [TN693.C9] 622s [669'. 3] 85-600046 



^ 



Page 



'V^ CONTENTS 

^__^ Abstract 

<^ Introduction 

■^Effects of impurities on copper alloys 

>^ Wrought alloys 

^S-, Cast alloys 

TjMetallurgical effects of tramp elements , 

/ ^ Hot workability 

Effect of lead , 

Effect of lead and iron , 

Effect of lead and zirconium , 

Fire cracking 

Effect of chemistry and processing , 

Liquid metal embrittlement , 

Additional studies 

Microstructural effects , 

Tin bronzes , 

Metallography 

Base alloy , 

Antimony , 

Iron , 

Lead , 

Nickel , 

Phosphorus , 

Silicon , 

Sulfur , 

Microprobe analysis , 

Iron , 

Nickel and antimony , 

Sulfur , 

Tin , 

Zinc , 

Grain size and shape , 

Eutectoid composition , 

Microsegregation , 

Failure analysis of bronze propellers , 

Composition , 

Results of analysis , 

W>Summary , 

t^ References , 



^ ILLUSTRATIONS 

, 1. Typical sorting routine for mixed copper alloy scrap 4 

2. Schematic representation of grain boundary voids due to dislocation 

impingement on grain boundary impurities 9 

3. Effect of lead, iron, and iron and lead on hot rolling of 60:40 brass 10 

, > 4. Effect of iron on hot rolling of a complex brass containing 1.33 pet Mn.... 10 

^ 5. Effect of lead and iron on hot rolling of a complex brass at 650° C 10 

6. Effect of lead and iron on hot rolling of a complex brass at 800° C 11 

7. Effect of zirconium and lead on hot twist ductility of a copper-nickel 

alloy 11 

8. Effect of lead on tensile ductility (reduction in area) of nickel-silvers.. 13 

9. Effect of lead on stress-strain behavior of nickel-silvers 13 



1 


2 


4 


6 


6 


8 


8 


9 


9 


11 


11 


12 


12 


13 


14 


14 


14 


14 


14 


14 


14 


14 


15 


15 


15 


15 


15 


15 


15 


15 


15 


15 


15 


16 


16 


16 


16 


17 


18 



11 



TABLES 



Page 



1. Typical copper alloys, nominal composition 3 

2. Compositional variations for wrought and cast copper alloys 5 

3. Limits for lead and bismuth in alpha copper alloys for hot rolling 6 

4. Effect of impurities on phosphor bronzes 7 

5. Effect of chemical composition on fire cracking 12 

6. Composition of tin bronze alloy used for microstructure studies 14 

7. Chemical composition of propeller blades 16 





UNIT OF MEASURE ABBREVIATIONS 


USED IN THIS REPORT 


HB 


Brinell hardness number 


pet percent 


in 


inch 


ton/in^ ton per square 
inch 


kg/mm^ 


kilogram per square 






millimeter 


wt pet weight percent 


mm 


millimeter 





METALLURGICAL EFFECTS OF IMPURITIES IN RECYCLED COPPER ALLOYS 

By Harry V. Makar ^ and William D. Riley^ 



ABSTRACT 

As part of a continuing research program for conserving domestic min- 
eral resources, the Bureau of Mines is investigating new and improved 
scrap identification techniques to improve sorted mixed scrap purity. 
This report focuses on various classes of wrought and cast copper alloys 
produced with recycled scrap. Based on a survey of the literature, 
principal alloys, such as the brasses and bronzes, are examined with 
respect to impurity elements and their effects on metallurgical be- 
havior. Effects of elements such as lead, antimony, iron, chromium, and 
aluminum are discussed. The metallurgical effects described include hot 
shortness, fire cracking, and undesirable phase transformations. 

'Branch Chief, Division of Ferrous Metals, Bureau of Mines, Washington, DC. 
^Physical science technician, Bureau of Mines, Avondale Research Center, Avondale, 
MD. 



INTRODUCTION 



Old scrap was 26 pet of the total 
2,278,000 metric tons of U.S. copper sup- 
ply in 1981 and is expected to be about 
29 pet by 2000 ( 4_) . 3 The smelters and 
refiners consumed 54 pet of the total, 
brass mills 40 pet, and others 6 percent. 
Overall, the secondary metals industry 
consists of an estimated 4,000 dealers 
and scrap processors. Details on second- 
ary copper are available in other publi- 
cations by the Bureau of Mines and the 
National Association of Recycling Indus- 
tries (NARI) (2, 2A, 23-24, 30-22). 

Specifications for the various alloys 
produced by the industry 'dictate the 
quality and the extent of upgrading and 
refining that may be required. In some 
cases, refining may be too costly or im- 
practical and the need for highly segre- 
gated scrap is essential. It has been 
estimated that at one time as many as 500 
commercial copper alloys were made in the 
United States. With such a large number 
of alloys and the almost infinite varie- 
ties of mixtures that can occur when 
these alloys come back into the recycling 
process, the task of proper upgrading is 
indeed monumental. This task is greatly 
lessened with the use of classifications 
such as those published by the National 
Association of Recycling Industries (24) . 
These classifications provide a standard- 
ized system for use by the industry to 
segregate mixed copper alloy scrap into 
recyclable categories. 

Several routine techniques are tradi- 
tionally used in the copper recycling in- 
dustry to identify scrap for effective 
segregation. These include identifica- 
tion based on object recognition, color, 
apparent density, magnetic attraction, 
and chemical spot tests. Some of the 
more sophisticated techniques commer- 
cially available include fluorescent X- 
ray spectroscopy, portable optical emis- 
sion devices, and thermoelectric sorters. 
Table 1 lists some common alloy groups 
and typical compositions compiled by the 
Institute of Scrap Iron and Steel (ISIS), 

■^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



and figure 1 shows a typical scheme for 
routine sorting of a mixture of copper 
alloys. These techniques, when properly 
applied by skilled sorters to mixtures of 
these alloys, permit effective identifi- 
cation and segregation into their respec- 
tive specified categories , such as red 
brass, yellow brass, manganese bronze, 
and aluminum bronze. 

Although traditional routine sorting 
methods are effective with regard to 
generic or descriptive specifications, 
there are many opportunities for intro- 
duction of impurities into new alloys 
made from improperly segregated scrap. 
Color distinctions may be obscured under 
certain lighting conditions. Also, there 
may be several specific alloys within a 
certain group, differing significantly in 
lead, antimony, aluminum, and other ele- 
ments that cannot be distinguished in 
routine sorting. In addition to the 
large number of alloys that must be han- 
dled, certain attachments to postconsumer 
parts also introduce impurities. These 
include iron fittings and plated parts. 
Previous Bureau of Mines research (2, 19- 
20 , 25 , 27 ) reported on improved tech- 
niques for metal scrap identification. 
The objective was to develop relatively 
simple, rapid techniques requiring few 
operator skills that would improve scrap 
purity during sorting. 

The purpose of this paper is to examine 
some of the reported metallurgical ef- 
fects of impurities in copper alloys. 
Based on the literature, a general over- 
view of reported effects is given for 
various impurities. This is followed by 
specific case studies in which the metal- 
lurgical effects of selected impurities 
are examined in more detail. The effects 
discussed include hot shortness, fire 
cracking, and unwanted phase transforma- 
tions. It should be noted that not all 
of the metallurgical effects are attrib- 
utable solely to composition. In some 
cases , the mechanisms of the problems en- 
countered are still not fully understood, 
and in other cases improper metallurgical 
processing in terms of foundry control 
and/or heat treatment may be more to 
blame than the presence of impurities. 



TABLE 1. - Typical copper alloys, nominal composition, weight percent 



Allov 


Cu 


Sn 


Pb 


Zn 


Others 


Common uses 


Silicon bronze. . 


82.5-97 


0-1 


Trace 


13 


1.35-4.50 Si 


Rod, bar, plate sheet, 
bearings, impellers, pump 
parts, valve stems, corro- 
sion-resistant castings. 


Red brass 


85 


5 


5 


5 


NAp 


General casting for good 
machining qualities, low- 
pressure valves, pipe fit- 
tings, small pump tast- 
ings, ornamental fixtures. 


Phosphorous 
bronze. 


95 


5 


NAp 


NAp 


NAp 


Bars, spings, rods. 


Navy brass 


88 


6-8 


0-2 


4 


NAp 


Steam pressure castings, 
valve bodies, pipe flange, 
gears and bushings, pump 
impellers, steam fittings, 
valves and parts. 


Yellow brass. . . . 


66 


1 


3 


30 


NAp 


Light casting not subject 
to high internal pressure, 
hardware fittings, orna- 
mental castings, gas 
cocks, radiator fittings. 


Muntz metal 


60 


NAp 


NAp 


40 


NAp 


Large nuts and bolts, braz- 
ing rods, condenser tubes. 


Admiralty brass. 


70 


1 


NAp 


29 


NAp 


Condenser tubes. 


Manganese bronze 


59 


.25 


.25 


38 


1 Fe 
1 Al 
0.5 Mn 


Valve stems, marine cast- 
ings and propellers , 
gears , bushings and 
bearings. 


Aluminum bronze. 


88 


NAp 


NAp 


NAp 


2 Fe 

10 Al 


Acid-resistant pumps, valve 
seats, gears, bushings and 
bearings. 


Copper-nickel. . . 


70 -90 


NAp 


NAp 


NAp 


10-30 Ni 


Boiler tubes, saltwater 
equipment. 


Nickel-silver. . . 


60 -65 


0-5 


0-7 


NAp 


5-25 Ni 


Hardware fittings, valves, 
plumbing fixtures, orna- 
mental castings, dairy and 
citrus machinery. 



NAp Not applicable. 

Source: Institute of Scrap Iron and Steel, Inc. 



Copper 
(red brown) 



Mixed scrap 



Visual 

(object recognition 

and color) 



Bronze and some brasses 
(dark yellow) 



Magnet 



Brasses (other than red brass) 
(light yellow) 



Aluminum bronze, 
manganese bronze 

(magnetic) 



Tin bronze, silicon bronze, 

leaded bronze, red brass 

(nonmagnetic) 



Admiralty metal, 

naval brass 

(white precipitate) 



Yellow brass, 
aluminum brass 
(no precipitate) 



AgNO, 



Manganese bronze 
(dark spot) 



Aluminum bronze 
(no reaction) 



Tin bronze 
(white precipitate) 



Silicon bronze 
(gelatinous material) 



Leaded bronze, 
red brass 



H,SO. 



HCI 

HNO, 

KOH 

Alizarin S 



Yellow brass 
(no color) 



Leaded bronze 
(white precipitate) 



Red brass 

(no white precipitate) 



FIGURE 1. - Typical sorting routine for mixed copper alloy scrap. 



EFFECTS OF IMPURITIES ON COPPER ALLOYS 



Aluminum brass 
(red color) 



A standard designation system for 
copper and copper alloys has been pub- 
lished by the Copper Development Associa- 
tion, Inc. (7^). Approximately 12 pages 
of wrought and cast copper alloys are 
listed under various families within the 
following four general classes: Brass- 
es, bronzes, copper-nickels, and nickel- 
silvers. Table 2 lists typical alloy 
families and their compositions. Many of 
these alloy families also have high-lead 
versions. Even within given families of 
alloys (e.g., brasses, bronzes, etc.), 
there are wide ranges for many elements 
that can cause metallurgical problems , 
singly or through interaction effects, if 



recycled scrap is improperly mixed. Most 
of these elements are intentionally added 
to impart certain desired properties in 
the original alloys. For example, anti- 
mony, arsenic, and phosphorus are added 
in small amounts to wrought alloys to 
inhibit dezincif ication; lead to enhance 
machinability and to promote smooth edges 
during shearing and blanking operations; 
tellurium, selenium, and sulfur for im- 
proved machinability; aluminum for re- 
sistance to impingement corrosion in 
condenser tubing alloys ; and chromium 
to form a heat-treatable alloy ( 18) . 
Some of these elements are also used in 
cast alloys for the same reasons. In 



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addition, lead is added to produce bear- 
ing alloys; iron, silicon, and aluminum 
may be used to provide improved mechani- 
cal properties; phosphorus and boron (as 
well as silicon) are used as deoxidizers 
( 18) . Mixing high-lead scrap with an 
otherwise similar alloy (dilution with 
cathode copper) would be particularly 
costly in alloy applications where hot 
workability demands very low lead con- 
centrations. In general, cast copper 
alloys can tolerate higher impurity lev- 
els than wrought alloys since they are 
not subjected to mechanical working. 
Typically, impurities in wrought alloys 
affect work response (e.g., hot and cold 
shortness) or thermal response (e.g., 
recrystallization, fire cracking), while 
in cast alloys impurities generally af- 
fect castability, soundness, and physical 
properties. 

WROUGHT ALLOYS 

Wrought alloys are generally hot-worked 
above the recrystallization temperature 
of the alloy and thus are free from the 
strain-hardening effects associated with 
cold working. Cold working is normally 
done below recrystallization tempera- 
tures. Foulger (12) has discussed im- 
purity effects on hot and cold worka- 
bility in copper alloys. In cold work- 
ing, impurities that affect the amount 
of the relatively hard and brittle beta 
phase must be carefully controlled. As 
a general rule, beta and alpha plus beta 
alloys are more easily hot worked, even 
in the presence of impurities, while 
single-phase alpha alloys require a much 
higher purity level. Hot shortness in 
wrought alloys is caused by such low- 
melting elements as lead, antimony, 
and bismuth and in some cases iron and 
silicon. Even very low levels of impuri- 
ties can be detrimental. Bismuth, for 
example, has been considered detrimental 
in amounts just sufficient to form a 
single atom layer along grain boundaries 
(6^, _12, 18) . Limits for lead and bismuth 
based on commercial hot-rolling experi- 
ence were reported by Jackson (17) for 
four common copper alloys as listed in 
table 3. 



TABLE 3. - Limits for lead and bismuth 
in alpha copper alloys for hot roll- 
ing, weight percent (17) 



Alloy 


Pb 


Bi 


70:30 brass 


0.02 
.015 
.004 


0.002 


Nickel-silver copper 

Phosphor bronze (95:5) 


.001 
.0004 



Cold shortness is a loss in ductil- 
ity in metals worked at temperatures 
below the recrystallization temperature, 
Glickman ( 14 ) has shown that cold short- 
ness is another intergranual phenomenon 
caused by extremely low amounts of bis- 
muth, tellurium, antimony, and boron seg- 
regated along grain boundaries. 

File cracking has also been attributed 
to low-melting impurities, such as lead, 
and those that form brittle intercrystal- 
line films, such as bismuth (6^, 18) . 
Copper alloys may alternately undergo 
heat-affected-zone (HAZ) cracking when 
lead, tellurium, bismuth, or sulfur is 
present, even though actual fire cracking 
does not take place (13) . 

Recrystallization and grain growth are 
also affected by impurities. For exam- 
ple, Fargette ( 11 ) has shown that, at 
least for relatively simple copper al- 
loys, a few hundred parts per million 
or less of silver, phosphorus, cadmium, 
manganese, or tin can increase the re- 
crystallization temperature. Recrystal- 
lization is inhibited because impuri- 
ties segregate in dislocations and grain 
boundaries, reducing their mobility (^, 
10). Impurities have also been shown to 
affect grain size and orientation ( 22 , 
^). 

CAST ALLOYS 

Cast copper alloys are used for sand 
casting or chill casting of various com- 
ponents. Typically, alloy additions are 
made to improve characteristics such as 
hardness or corrosion resistance. Usual- 
ly one characteristic is improved at the 
expense of another. For example, iron 
is added to some yellow brasses to in- 
crease strength at the expense of reduced 
elongation. Table 2 lists the range of 
compositions for some principle cast- 
ing alloys. Elements such as lead and 



antimony, which are in relatively high 
concentrations in some alloys, could not 
be tolerated in others. Cast alloys gen- 
erally can tolerate greater amounts of 
impurities than wrought alloys. The ef- 
fect of impurities on cast alloys may al- 
so depend on the method of casting, i.e., 
whether sand cast or chill cast (32) . 
Mechanical properties can be impaired in 
certain tin bronzes by magnesium, sili- 
con, aluminum, and even zinc. These ele- 
ments have similar properties in that 
they form oxide films on the surface of 
molten bronzes which break up on pouring 
and cause rough surfaces on the resulting 
chill-cast products. Because film forma- 
tion interferes with feeding within 
the mold, fissure porosity can result, 
greatly reducing mechanical properties 
(33) . Proper segregation of scrap alloys 
is also essential in that some silicon 
bronzes contain small amounts of arsenic 
and antimony. In bronze castings, arsen- 
ic and antimony decrease the solubility 
of tin, thus increasing and coarsening 
the brittle delta phase, which impairs 
the mechanical properties (1-^). Arsenic 
and antimony concentrations as low as 
0.12 and 0.18 wt pet were found to se- 
verely reduce the tensile elongation of 
tin bronzes (32) . 



In general, the impurities tend to de- 
crease the mechanical properties and 
castability. It is also possible that 
impurities such as aluminum and silicon 
can cause casting voids, which serve as 
sites for stress corrosion cracking. 

Similar effects of impurities on phos- 
phor bronzes , tin bronzes , and their 
leaded alloys have been reported by Win- 
terton ( 33 ) and Stolarczyk (32). For 
example, table 4 illustrates the effects 
of aluminum, silicon, iron, lead, and 
bismuth on a typical phosphor bronze. In 
general, an increase in impurities de- 
creases tensile strength and elongation, 
while hardness is variable. 

Impurities can also cause microstruc- 
tural changes in cast copper alloys. For 
example, grain shape or size can be 
changed, as in the case of iron in brass- 
es (10). In other cases, solid solutions 
or compounds are formed. It has also 
been shown (26) that chromium, although 
below specified maximum concentrations, 
can lead to fatigue failure in a 
manganese-nickel-aluminum bronze alloy 
(75Cu-12Mn-8Al-2Ni-3Fe). Failure results 
from chromium segregation and eventual 
formation of a dendritic phase, leading 
to coarse grains and weak structure. 



TABLE 4. - Effect of impurities on phosphor bronzes (33-34) 





Concentration, 
wt pet 


Tensile strength, 
ton/in^ 


Elongation, 
pet 


Hardness , 
HB 


Standard alloy. . 
Aluminum 

Bismuth 


NAp 

0.005 
.01 
.47 
.32 

1.25 
.28 
.80 

1.69 
.24 
.70 

1.61 
.01 
.07 
.38 


27.2 
25.3 
21.8 
23.2 
25.6 
23.8 
28.5 
26.1 
22.2 
26.3 
25.4 
23.8 
26.3 
24.3 
24.2 


18 

15 

5 

6 

14 

10 

15 

11 

4 

15 

15 

10 

18 

9 

7 


138 
123 
133 
130 
127 


Iron 


126 
136 


Lead 


132 
139 
128 


Silicon 


125 
126 
119 




131 
138 



NAp Not applicable. 



METALLURGICAL EFFECTS OF TRAMP ELEMENTS 



The practical aspect of secondary cop- 
per alloy production is presumably based 
on achieving the best corrosion resist- 
ance at the lowest cost. Cost in this 
context refers to both processing and raw 
material cost. The extent to which rela- 
tively low-cost scrap can be utilized to 
achieve required final quality without 
excessive processing cost is well under- 
stood by the industry. Millions of tons 
of high-grade products testify to the 
metallurgical acceptability of recycling. 
As noted in the above overview section, 
the effects of certain undesired elements 
are well understood, and for the most 
part they are effectively avoided. Some 
metallurgical problems have arisen, how- 
ever, where interaction effects among 
various elements and/or certain micro- 
structural behaviors were not well under- 
stood. The following case studies were 
selected as examples of the perhaps less 
well known metallurgical effects of im- 
purities in copper alloys and to empha- 
size the importance of careful scrap 
sorting and segregation. Some of the 
examples also illustrate that corrective 
metallurgical treatments may counteract 
certain impurities. It is important to 
note that although impurities are often 
major problems, certain metallurgical 
problems are due to factors other than 
composition. 

HOT WORKABILITY 

Metallurgical and processing factors 
associated with the hot workability of 
brasses, bronzes, copper-nickels, and 
nickel-silvers have been examined by 
a number of researchers. Studies by 
Cook (6^) , Davies (^) , Foulger ( 12 ) , and 
Heslop (15) were selected for most of the 
following discussion. The preceding dis- 
cussion described various effects of al- 
loying elements and impurities on copper 
alloys. It is important to note that the 
literature from which these data were 
obtained also mentions the use of metal- 
lurgical controls (e,g, , special alloy 
additions or processing techniques) to 



enhance hot workability. Even without 
harmful levels of impurity elements, im- 
proper metallurgical processing can cause 
hot shortness. 

Factors affecting hot workability in- 
clude crystal structure, soundness, seg- 
regation, and composition. The effect of 
composition is highlighted here to illus- 
trate some of the direct or potential 
effects of certain impurity elements. 
Alloys such as brasses, aluminum bronzes, 
copper-nickels, and silicon bronzes may 
contain up to 1 or 2 pet of various al- 
loying elements to achieve desired 
mechanical properties. Except for lead 
and other low-melting elements, the al- 
loying elements enter into solid solution 
and generally have little or no effect on 
hot workability. Interaction effects 
from impurities at relatively low concen- 
trations can have severe detrimental ef- 
fects, however, although there seems to 
be little published on this subject. It 
is important to note that the quality of 
the cast ingot prior to working and 
response to homogenizing soak treatments 
are extremely important factors affecting 
hot working, A fine, uniform cast grain 
structure, for example, generally results 
in better hot workability and tolerance 
for impurities than a coarse, nonuniform 
grain structure. The severity of hot 
working also determines hot workability, 
such that a given alloy may fail during 
hot piercing for tubemaking but form sat- 
isfactorily during hot rolling or extru- 
sion. Regarding workability of metals in 
general, Semiatin (29) gives an excellent 
description of the action of dislocations 
and intergranular impurities on the for- 
mation of voids at grain boundaries. 
Secondary tensile stresses develop at the 
voids during metal working operations, 
eventually leading to fracture. Figure 2 
is a schematic description by Semiatin 
showing grain boundary voids and disloca- 
tion impingement (inverted T's). Figure 
2 also depicts triple-junction cracks, 
which appear at the junction of three 
grains and subsequently lead to fracture 
during hot working. 




Triple-junction 
crack 



Grain-boundary voids 

FIGURE 2. - Schematic representation of grain 
boundary voids due to dislocation impingement on 
grain boundary impurities. Also shown is a triple 
junction crack (29). 

Effect of Lead 

Lead is among the most prevalent and 
deleterious elements affecting hot worka- 
bility. The maximum that can be toler- 
ated depends on such factors as overall 
composition, structure, and processing 
techniques. 

In brasses, lead is harmful at levels 
as low as 0.01 wt pet. In phosphor 
bronzes, Jackson (J_7) showed that 0.004 
wt pet lead is considered the maximum 
tolerable for hot rolling (table 3). In 
other alloys, lead may be tolerated up to 
0.02 wt pet, and even 0.05 wt pet or 
higher. Extreme care is needed if alloys 
containing lead for machinability are to 
be hot-worked. For copper-nickel alloys, 
bismuth, tellurium, and selenium must be 
limited to levels as low as 0.001, 0.003, 
and 0.006 wt pet, respectively (12). The 
harmful effects of lead (and the other 
low-melting elements) are attributed to 
the general characteristics of low melt- 
ing point and limited solid solubility in 
the parent alloy. The lead and/or its 
low-melting compounds segregate along 
grain boundaries and are liquid at normal 
hot-working temperatures. Hot shortness 
may or may not occur, depending on inter- 
facial tensions between the liquid phase 



and the solid grains of the parent alloy 
(_5, j7, 33.). The details of this pro- 
posed mechanism are beyond the scope of 
this paper. As stated earlier, the ef- 
fect of such impurities may be enhanced 
or diminished, depending on other factors 
such as grain size control, other alloy- 
ing elements, initial cast structure, and 
processing techniques. 

Effect of Lead and Iron 

Figure 3 shows the effect of high lev- 
els of lead on edge cracking of a 60:40 
brass during hot rolling. The alloy used 
in this study was relatively pure except 
for lead and showed appreciable cracking 
at 0.3 wt pet Pb and catastrophic crack- 
ing at 1.0 wt pet Pb. The addition of 
1.0 wt pet Fe increases the tolerance for 
lead. 

Studies by Davies ( 10 ) described the 
beneficial effects of iron in lead- 
containing brasses and offered a possible 
explanation. Figure 4 shows the effect 
of iron on hot rolling of brass alloys 
containing approximately 0.8 wt pet Pb 
(plus 1.3 wt pet Mn and 1.5 wt pet Al). 
Figures 5 and 6 show the effect of iron 
in a somewhat different manner, i.e., two 
iron levels and varying concentrations of 
lead. The actual mechanism by which the 
beneficial effect of iron occurs is 
apparently not fully understood. Davies 
refers to other investigators who studied 
cast beta brasses containing 3 wt pet or 
more aluminum and related the intererys- 
talline fraction to the segregation of 
aluminum atoms along grain boundary ar- 
eas. They felt that aluminum was being 
replaced by iron atoms, which were less 
harmful. In the alloys studied by Davies 
and depicted in figures 4, 5, and 6, the 
iron was thought to replace the lead 
atoms. A typical composition of these 
brass alloys is, in weight percent, 60 
Cu, 1 Sn, 1.5 Al, 0.2 Ni , 0.8 Pb, balance 
Zn. Iron, manganese, and lead concentra- 
tions ranged between 0.04 and 1.34, 0.44 
and 1.54, and 0.05 and 1.16 wt pet, 
respectively. 



10 





pet Pb 



1.0 pet Fe 





0.3 pet Pb 



1.0 pet Fe and 1.0 pet Pb 




1 .0 pet Pb 

FIGURE 3. - Effect of lead, iron, and iron and lead on hot rolling of 60:40 brass (6). 



80 



70 



60 



50 



40 



30 



20 



10 



Not cracked CD DD 



KEY 

Rolling temperature 

650° C 800° C 

■ D 




0.2 0.4 0.6 0.8 

IRON, wt pet 



1.2 



1.4 



FIGURE 4. - Effect of iron on hot rolling of a 
complex brass containing 1.33 pet Mn at 650° and 
800° C (10). 




0.4 0.6 0.8 1.0 

LEAD, wt pet 

FIGURE 5. - Effect of lead and iron on hot 
rollingof acomplexbrassat 650°C (JO). 



1.2 



11 



80 



70 



60 - 



50 - 



40 - 



I- 30 



20 - 



10 




0.2 



0.4 



0.6 0.8 

LEAD. w1 pet 



1.4 



FIGURE 6. - Effect of lead and iron on hot 
roiling of a complex brass at 800 C (10). 



Effect of Lead and Zirconiutn 

Figure 7 illustrates the severe effect 
of lead on hot ductility of a copper- 
nickel, and also shows how this effect 
can be offset by zirconium additions 
( 16) . Other studies indicate that ap- 
proximately 0.05 wt pet Ce and other rare 
earths can improve the hot workability of 
copper alloys. The beneficial effects 
of zirconium are presumably due to pro- 
nounced grain refinement, while rare 
earths, and certain other elements (e.g., 
thorium, uranium, and lithium) form 
high-melting intermetallic compounds with 
lead. 

FIRE CRACKING 

Fire cracking is a for^ of embrittle- 
ment (cracking) that occurs during rapid 
heatup to annealing temperatures. This 
type of cracking has been the subject of 
a number of studies, yet the mechanism is 
not fully understood. In one study by 
Sato (28) fire cracking was investigated 
for aluminum brass, copper-nickel, and 




700 



FIGURE 7. 



800 



1,000 



900 

TEST TEMPERATURE, "C 

Effect of zirconium and lead on hot twist ductility of a copper-nickel alloy (16) 



1,100 



12 



chromium-copper. It was concluded that 
embrlttlement occurred as a result of 
voids along grain boundaries, similar to 
the behavior observed for alloys subject- 
ed to high-temperature creep and tensile 
tests. The embrlttlement due to void 
formation was further found to be closely 
related to several factors, including — 

1. Magnitude of the residual stress 
from cold working. 

2. Heating rate. 

3. Time at temperature. 

4. Annealing temperature. 

5. Grain size. 

6. Reduction ratio during cold working. 

The effects of impurities were not 
described, implying that they were 
minor compared with the above-mentioned 
factors. 

Effect of Chemistry and Processing 

Isler (16) undertook a detailed study 
to define the mechanism of fire cracking. 
Table 5 shows results of preliminary 
tests relating chemical composition to 
fire cracking. The alpha alloys with 
lead were clearly prone to fire crack- 
ing, whereas the presence of beta phase 
reduced the tendency. In the absence 
of nickel, cracking could not be in- 
duced. All cracks were intercrystalline. 
A large number of tests conducted on 
alloy A revealed that this alloy could 
exist in a sensitive (to fire cracking) 
as well as a nonsensitive state after be- 
ing cold-worked and heat-treated in pre- 
sumably identical fashion. This observa- 
tion led to the conclusion that in addi- 
tion to lead concentration and type of 
crystal structure, other factors governed 
sensitivity to fire cracking. Further 
detailed studies were conducted to exam- 
ine effects of residual stresses from 
cold work, the rate of heating, grain 
size, the role of porosity, and the role 
of lead. Casting porosity was clearly a 
major factor rendering alloys sensitive 
to fire cracking. The effect of lead was 
studied in considerable detail, providing 
the major subsequent observations on 
which a proposed mechanism was based. 



TABLE 5. - Effect of chemical composition 
on fire cracking (16) 





Composition, 


Phase(s) 


Sensitive 


Alloy 


wt pet 


present 


to fire 




Cu 


Zn 


Ni 


Pb 


cracking 


A 


62 


19 


18 


1 


a 


Yes 


B 


62 


20 


18 





a 


No 


C 


62 


24 


13 


1 


a 


Yes 


D 


53 


38 


8 


1 


a 


Yes 


E 


47 


41 


10 


2 


a and p 


No 


F 


51 


42 


6 


1 


a and p 


No 


G 


71 


28 


ND 


1 


a 


No 



Liquid Metal Embrlttlement 

Hearing tests on alloys A and D, table 
5, revealed that both alloys cracked at 
318° C, slightly lower than the melting 
point of lead (327° C) . Microprobe anal- 
ysis showed that lead particles contained 
the main alloying elements of the matrix 
(copper, zinc, and nickel). This and the 
fact that lead is known to form low- 
melting eutectics with these elements 
(e.g., 318° C with 0.5 wt pet Zn) sup- 
ported the conclusion that the fire- 
cracking temperature coincided with the 
melting point of the lead particles. The 
fire-cracking temperature was found to be 
independent of the matrix composition. 
Liquid metal embrlttlement was thus sus- 
pected as being responsible for fire 
cracking, and a series of tensile tests 
were run to show whether alpha nickel- 
silver and lead formed an embrittling 
couple. Figure 8 shows tensile ductility 
(reduction of area) for alloys A, B, and 
F at various temperatures. These results 
(and supporting Charpy impact tests) 
showed a marked drop in ductility at 
about 300° C for alloy A, but not for 
alloy B (which is the same as alloy A but 
without lead). A ductility minimum also 
occurred at about 300° C for all alloys, 
owing to spontaneous strain aging for 
the alpha-phase alloys (A and B) and 
to intercrystalline cavity formation for 
the alpha-beta alloy F. Figure 9 shows 
strain-aging embrlttlement as evidenced 
by the serrations in the stress-strain 
curve of alloy B but shows no serrations 
for alloy F. 



13 



z 
o 

t- 

o 

z> 
a 
111 

CO 



80 



70 



60 - 



50 



40 



30 - 



Q~~ 



\:>^ 



XK 



.^ 



\ 



20 



10 



\ 
\ 
\ 
\ 

KEY 

Alloy A.awith Pb 
□ Alloy B. a without Pb 
■ Alloy F, a* /3 with Pb 



^1 






c 
"5 

Q. 
O) 

c 



/ 




100 200 300 400 

TEST TEMPERATURE. «C 



500 



FIGURE 8. - Effect of lead on tensile ductility 
(reduction in area) of nickel-silvers (16). 

Microprobe analysis was also conducted 
to show that there is a marked concentra- 
tion of lead along the fissures of the 
advancing crack front during fire crack- 
ing. This further supported the liquid- 
metal embrittlement model. 

Additional Studies 

Other tests were conducted to study 
stress relaxation and desensitizing phe- 
nomena, the latter being related to dis- 
locations piling against grain boundar- 
ies. The influence of storage time after 
cold work was also examined, as was the 
effect of lead on alloys of alloy A 
composition but with 0,2 and 2.5 wt 
pet Pb. Both these and the previously 
discussed results were related to the 
Griff ith-Orowan fracture theory, leading 






I- 

V) 



ou 




\ 










/ 


20 


1 


/ 


A 


Alloy F 




10 


H 






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- 




lo 


O 


o 


O ~~^ N^ 




n 


1 o 

/ o 

f ^ 


o 

o 


o 

o 
t 

CO 


"" 1 





STRAIN 

FIGURE 9. - Effect of lead on stress-strain be- 
havior of nickel-silvers (alloy B; alpha alloy plus 
lead; alloy F; alpha plus beta alloy plus lead) 
(16). 

to the following summary characteristics 
associated with alloys sensitive to fire 
cracking: 

1. A second phase is present that 
melts during heat up, forming an embrit- 
tling couple with the alloy. 

2. The melting point of the second 
phase must be low enough so that the re- 
sidual stresses are relaxed only slightly 
when melting occurs. 

3. High yield strength of the matrix 
alloy permits substantial buildup of the 
residual stresses. 

4. The alloy must contain voids along 
the potential fracture path, thus elim- 
inating the need for crack initiation. 

It was not clear from Isler's study 
why the alloy with lead but no nickel 
(alloy G) was not sensitive to fire 
cracking. 



14 



MICROSTRUCTURAL EFFECTS 



Micros true tural effects, such as solute 
element segregation at grain boundaries 
and phase relationships , were considered 
to some extent in the above discussions 
on hot workability and fire cracking. 
Other reported studies concentrated on 
microstructural evaluations to deter- 
mine the effects of various elements on 
the micros true ture of tin bronze and 
to determine the cause of failure of 
manganese-nickel-aluminum bronze cast 
propeller blades. 

TIN BRONZES 

Couture (9^) studied a tin-bronze alloy 
(88 Cu-10Sn-2Zn) to determine the effects 
of nickel, phosphorus, iron, lead, anti- 
mony, sulfur, and silicon on microstruc- 
ture. The alloy base composition and 
ranges of additions studied are listed 
in table 6. The purpose was to produce 
microstructures that would help explain 
the influence of these elements on the 
properties of tin bronzes investigated 
by other researchers. The observations 
made from micrographs are highlighted 
here. 

TABLE 6. - Composition of tin bronze 
alloy used for microstructure 
studies, weight percent (8) 



Element 



Cu. 
Sn. 
Zn. 
Fe. 
Ni. 
P.. 
Pb. 
S., 
Sb. 
Si. 



Master alloy 


Range of 




impurities 


87.0 


(') 


10.4 


(') 


2.4 


(0 


.017 -0.018 


0.02- 1.88 


.0015- .0018 


.002-3.38 


.003 - .004 


.004-1.05 


.004 - .005 


.004-1.84 


.004 - .005 


.004- .030 


.0016 


.002-2.19 


.002 


.002- .035 



'The master alloy was used to make all 
subsequent melts with the various levels 
of impurities. 



Metallography 

Base Alloy 

Microstructure consisted of the contin- 
uous copper-rich alpha phase (tin and 
zinc in solution) and pools of the alpha- 
delta eutectoid of tin bronzes. 

Antimony 

At 0.78 and 2.19 pet, antimony caused 
an increase in the amount and size of the 
delta phase. This was attributed to de- 
creased solubility of tin as antimony 
goes into solid solution. Grain size was 
not affected. 

Iron 

At 1.88 pet Fe , the eutectoid pools in 
the cast structure were more abundant and 
coarser. The as-cast high-iron alloys 
also showed a fine precipitate around the 
eutectoid and a starlike constituent in 
the eutectoid, both presumably iron-rich. 
Heat treatment at 700° C (1,292° F) dis- 
solved the delta phase and resulted in 
dense precipitation of a fine, iron-rich 
constituent throughout the matrix. Some 
grain refinement occurred, ranging from 
radially columnar at 0.48 pet Fe to equi- 
axed (0.5-mm-diameter) at 1,88 pet Fe, 

Lead 

Delta phase and grain size were not af- 
fected by lead additions. However, be- 
cause of low solubility, lead particles 
concentrated in the eutectoid. 

Nickel 

Additions up to about 1 wt pet had no 
significant effect. At the 1.33- and 
3.38-wt pet levels, however, the micro- 
structure showed a more abundant delta 
phase in large clusters than was observed 
in the base copper-tin-zinc alloy. Alpha 



15 



pools within the eutectoid were larger. 
Theta phase, though observed in nickel- 
containing bronzes by others, was not 
present in the alloys studied by Couture. 
Possible explanations for this difference 
are the absence of lead, lower nickel 
contents, and lack of equilibrium in the 
test specimens. Grain shape and size 
were also modified by nickel. Grains 
changed from radially columnar 3/8 in 
(9.5 mm) long to equiaxed grains 1/64 in 
(0.8 mm) in diameter at 3.38 wt pet 
nickel. 



Iron 

Iron was concentrated in the matrix. 
The iron concentration in the eutec- 
toid was well below that of the sample 
average. 

Nickel and Antimony 

These were primarily associated with 
the eutectoid. 

Sulfur 



Phosphorus 

Phosphorus contents of 
pet resulted in an intermet 
presumed to be copper pho 
which at low concentrations 
uted mainly on the outside 
alpha and delta eutectoid, 
concentrations was distribu 
eutectoid and in adjacent 
matrix alpha phase. Phos 
effect on grain size. 



.1 to 1.05 wt 

allic compound 

sphide (CU3P) , 

was distrib- 

edges of the 
and at higher 
ted within the 

areas of the 
phorus had no 



Larger inclusions were confirmed as 
sulfides. 



Tin 



Tin concentration in the matrix was be- 
low the alloy average. In the cored ar- 
eas, it was similar to the average compo- 
sition. The eutectoid showed about three 
times the average tin composition. 

Zinc 



Silicon 

Silicon additions resulted in a more 
abundant and coarser eutectoid. Grain 
size changed from columnar to equiaxed 
even at the low addition (0.005 pet). 
Grain size was reduced only slightly with 
further additions. 

Sulfur 

At all levels of addition, sulfur 
formed a large number of translucent dark 
gray, probably complex, sulfides. Grain 
size was not affected. 



Matrix and cored areas were about the 
same, owing to rapid zinc diffusion, 
which was greater than in the eutectoid. 

Grain Size and Shape 

Grain structure was changed from colum- 
nar to equiaxed with additions of nickel, 
iron, or silicon. The grain refinement 
achieved was much less than that obtained 
in other studies with zirconium. Grain 
size and shape were not affected by phos- 
phorus, lead, antimony, or sulfur. 

Eutectoid Composition 



Microprobe Analysis 

Microprobe analyses were conducted to 
determine the distribution of the impu- 
rity elements. Areas examined included 
the matrix (center of the dendrite arms), 
coring (surrounding the eutectoid), delta 
phase (or eutectoid pools in the cases of 
very small delta areas), and inclusions. 
The following qualitative results were 
obtained. 



The binary copper-tin phase diagram 
predicts alpha plus epsilon as the equi- 
librium phases at room temperature for 
tin bronze alloys. The eutectoid decora- 
position of delta to alpha plus epsilon 
(350° C) is sluggish, however, and delta 
is retained down to room temperature. 
Although the delta would be expected to 
contain 32.6 pet Sn, the presence of more 
than 2 pet Zn and additional elements 
will alter the delta composition. 



16 



Microsegregatlon 

Metallographic examination revealed 
that phosphorus and lead segregated in or 
near the eutectoid as compounds or solid 
solutions and were present in the matrix 
between dendrite arms, thus confirming 
they segregated in the last-to-f reeze 
liquid. 

FAILURE ANALYSIS OF BRONZE PROPELLERS 

As part of an extensive failure analy- 
sis program conducted on large propeller 
blades, Raymond (26) performed detailed 
microstructural analysis to determine 
cause of failure. Two 6-ton cast propel- 
ler blades failed while in service on a 
U.S. Coast Guard icebreaker. The alloy 
was a manganese-nickel-aluminum bronze, 
selected for excellent mechanical prop- 
erties and good erosion and corrosion 
resistance in high-velocity seawater. 
Foundry and welding characteristics of 
the alloy are superior to those of con- 
ventional aluminum bronzes. When proper- 
ly alloyed, a stable microstructure of 
approximately 50 pet alpha and 50 pet 
beta is obtained. 

Composition 

Chemical analyses were compared among 
samples from the failed blades and a 
good blade, and against the specification 
for MIL-B-21230 A, alloy 2 (table 7). 
The compositions all appeared to agree 
rather closely. However, a slightly 
higher concentration of chromium was 
noted in the samples from the failed 
blades. Although within the allowable 



maximum for "others," this small amount 
of chromium resulted in microsegregatlon, 
which led to an unstable microstructure, 
also causing a degradation of mechani- 
cal properties. For example, tensile 
strength, elongation, and Charpy V-notch 
energy were respectively 25, 50, and 75 
pet lower than typical values. 

Results of Analysis 

The study reported by Raymond ( 26 ) 
involved a detailed analysis includ- 
ing microstructural analysis, mechanical 
property evaluation, thermal analysis, 
and fracture mechanics. The overall ob- 
jective was to devise a corrective weld 
repair and heat treatment that would 
permit salvaging additional propeller 
blades. This was successfully accom- 
plished. The overall analysis was rather 
detailed and beyond the scope of this 
paper. The studies to define the micro- 
structural effects of chroDd-um are high- 
lighted here. 

Chromium was isolated as the deleteri- 
ous contaminant which formed an iron-rich 
dendritic phase ("sparkle") during solid- 
ification from the melt. A detailed ion 
microprobe mass analysis on a dendrite 
showed — 

1. Dendrite was composed of iron, 
chromium, manganese, and only a small 
amount of nickel. 

2. Surrounding phases were discontinu- 
ous alpha, richer in copper than the dis- 
continuous beta, which was richer in man- 
ganese and iron. 

3. Aluminum and nickel were uniformly 
distributed between the alpha and beta 
phases. 



TABLE 7. - Chemical composition of propeller blades, weight percent (26) 



Element 


Failed 
blade 1 


Failed 
blade 2 


Good 
blade 


MIL-B-21230A 
(alloy 2) 


Cu 


74.09 

11.90 

7.77 

.015 
3.42 
2.42 
<.004 
.027 



74.11 

11.80 

7.82 

.016 
3.28 
2.59 
<.0078 

.041 



74.30 

11.95 

7.90 

.005 
3.10 
2.51 


.06 



71 (min) 


Mn 


11-14 


Al 


7.0-8.5 


Cr 


Not specified 
2.0-4,0 


Fe 


Ni 


1.5-3.0 


Pb 


0.03 (max) 


Si 


0.10 (max) 


Others 


0.05 (max) 



17 



Based on these results, it was con- 
cluded that trace amounts of chromium 
caused the formation of iron-chromium- 
manganese dendrites at high temperatures. 
The surrounding matrix was thus depleted 
of (1) the manganese necessary to sup- 
press eutectoid decomposition and (2) the 
iron necessary for fine grain structure. 
Manganese suppresses eutectoid decomposi- 
tion because, being soluble in copper, it 



lowers the melting point of the alloy. 
The decomposition of eutectoid beta re- 
sults in a brittle ternary phase, ob- 
served as fine lamellar precipitates in 
the beta phase. Two detailed heat treat- 
ments were devised for restoration of 
mechanical properties, both requiring 
that the eutectoid temperature of 675° C 
(1,250° F) be exceeded. 



SUMMARY 



The role of recycling copper alloys has 
been examined with regard to known metal- 
lurgical effects that result from exces- 
sive concentrations of alloying elements 
or impurities. A profile of the industry 
is briefly presented, followed by an 
overview of the reported metallurgical 
problems. A few of the problems were ex- 
amined in more detail (case studies) for 
explanations of possible mechanisms and 
corrective procedures. 

Wrought copper alloys are generally 
sensitive to even minor amounts of low- 
melting elements such as lead, bismuth, 
and antimony. Depending on the impurity, 
harmful effects have been observed for 
concentrations as low as 0.004 pet, al- 
though considerably higher concentrations 
can be tolerated for many of the alloys 
under appropriate processing conditions. 
The predominant effects are hot and cold 
shortness and fire-cracking tendency due 
to grain boundary segregation of such 
elements. Sensitivity to impurities is 
related to phase relationships, with 
single-phase alpha alloys being most sen- 
sitive. Certain other elements, includ- 
ing iron and silicon, may produce harmful 
effects, but in the case of iron can also 
counteract the harmful effects of low- 
melting impurities. Increased tolerance 
for otherwise harmful impurities is 
achieved also with additions of zirconi- 
um, rare earths, or uraniijm. 

Cast alloys are considerably more 
tolerant of impurities than wrought al- 
loys, although various impurities can 
have pronounced effects on castability 
and mechanical properties. Some of the 
elements known to adversely affect prop- 
erties if not closely controlled include 
manganese, silicon, aluminum, iron, 
arsenic, antimony, lead, and bismuth. 



Small amounts of chromium (0.015 wt pet) 
caused serious failures in cast aluminum 
bronzes owing to preferential segregation 
and subsequent effects on transformation 
kinetics and brittle behavior. 

Although the effects of impurities have 
been well defined for many alloy systems 
and adequate information seems to be 
available to reevaluate alloy specifica- 
tions, it is important to note that 
definitive information is not available 
for all cases, owing to the complexity of 
some alloys and the interaction effects. 
The many processing variables and service 
conditions further prevent an understand- 
ing of impurity effects in commercial 
alloys. 

A major objective of the Bureau of 
Mines recycling research has been to 
improve the efficiency of recycling by 
introducing new technologies to more 
accurately identify and sort scrap metals 
(2, 19-20, 27), thereby minimizing the 
chance of introducing tramp elements. 
The evidence clearly supports the often- 
expressed concern that scrap must be 
carefully and accurately segregated to 
avoid harmful impurity effects when re- 
cycled, but all metallurgical problems 
are not attributable solely to impuri- 
ties. Care must also be exercised to 
assure that established metallurgical 
treatments are followed when processing 
recycled alloys and when putting them in- 
to service. Unfounded claims that pro- 
cessing or service failures are caused 
solely by impurities from scrap do not 
effectively serve the recycling industry, 
the scrap user, or the Nation when con- 
servation measures such as recycling are 
so vitally needed in extending our min- 
eral resources. 



18 



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19 



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